Polyimide and polybenzoxazole, featured by their excellent heat resistance and insulating performance, have been used typically for insulating layers in semiconductor devices.
Another practice is to use a pre-cyclized precursor (polyimide precursor or polybenzoxazole precursor) that is more soluble into solvent, in place of polyimide and polybenzoxazole, to apply it onto a substrate or the like, and to cyclyze the precursor under heating to thereby form a cured film.
As this sort of polyimide precursor, Patent Literature 1 discloses a negative photosensitive resin composition that contains 100 parts by mass of polyimide precursor with a specific structure; and (B) 0.1 parts by mass to 20 parts by mass of a photo-polymerization initiator.
Patent Literature 2 discloses a polyimide precursor varnish that includes a composition containing a polyimide precursor and a solvent, the polyimide precursor being obtained by polycondensation of a diamine with an acid dianhydride; containing 27 to 87 mol % of diamine component A represented by Chemical Formula (A) and 7:3 to 13 mol % of diamine component B represented by Chemical Formula (B), both relative to the total diamine; the diamine component A and the diamine component. B totaling up to 80 mol % or more of the total diamine; the acid dianhydride being a pyromellitic dianhydride represented by Chemical Formula (C); the diamines totaling up to 47.5 to 52.5 mol % relative to the total of the diamines and the acid dianhydride; and the acid dianhydrides totaling up to 52.5 to 47.5 mol %.

Patent Literature 1 also describes that the composition can contain 0.1 to 20% by mass of a monomer or so represented by Chemical Formula (D), relative to the total solid resin content of the polyimide precursor.
